They concluded that folic acid lowers the concentrations homocysteine in the blood. High amounts of homocysteine have been linked to the increased risk of stroke, cardiovascular disease and deep vein thrombosis.

Despite the link between homocysteine and other cardiovascular diseases, researchers were unable to determine whether folic acid would cut the risk for diseases other than stroke.

They were able to conclude that the use of folic acid supplements reduced the risk of stroke by an average of 18 percent. In subgroup analyses, an even greater reduction of risk was seen when the treatment lasted over 36 months, which resulted in a 29 percent reduced risk.

Researchers also found that if folic acid was able to cut the concentration of homocysteine in the blood by more than 20 percent, the risk was reduced by 23 percent and, if the patient had no previous history of stroke, the risk was cut by 23 percent.

The study also found that in areas of the country that did not already have supplementation through fortified or partly fortified grain, folic acid supplementation decreased the risk of stroke by 25 percent.

The authors said future studies were needed to test the efficacy of folic acid and that clinical trials should be done over longer periods of times — four years or more; in regions without grain fortification; and on individuals that have a history of stroke.